In clinical X-ray imaging, for instance in mammography, quantitative imaging is used to measure the amounts of different types of material, such as tissue, which may be used as input for improved risk assessment, detection, diagnosis, and treatment. For example, measurements of the volumetric breast density using quantitative imaging allows assessing the risk of developing breast cancer, assessing the efficiency of mammography, deriving improved radiation dose estimates, and monitoring the effect of drug treatment over time in order to adapt the drug dosage. Another application of quantitative imaging in mammography is to differentiate between different lesion types, for instance, cystic and solid lesions, in order to provide a better diagnosis.
Several methods for quantitative imaging are available, including spectral and non-spectral imaging methods. Spectral imaging refers to imaging an object at multiple photon-energy spectra, for example at least two different energy spectra. In WO 2013/076662, a method is described for spectral image processing in X-ray imaging. At a given dose, spectral imaging is generally associated with higher noise compared to non-spectral imaging because 1) the available dose is split over several spectra, and 2) the spectral processing often involves taking a difference between the different spectra, which is an operation that increases noise. The increased noise generally leads to a lower limit on the structure size, which can be detected and quantified. In contrast to spectral imaging, non-spectral quantitative imaging methods often rely on a-priori information and additional assumptions, or full non-spectral 3D image data such as acquired by computed tomography.
Quantitative imaging is often applied on a global level, reporting for instance a single value for an image or the entire breast. However, local quantitative information, for instance a map of material properties over a breast, can be valuable, for example to find regions of increased risk, for better differentiation in treatment monitoring, to classify or quantify local material properties, such as prevalence of certain materials, including tumor tissue, cyst fluid, and contrast medium. Local quantitative information obtained with spectral imaging may, however, be too noisy to be useful at reasonable dose levels. While quantitative information obtained with non-spectral imaging is less noisy at a given dose, the inherent need for assumptions and/or a-priori information often leads to lower overall precision of the measurement compared to spectral imaging.
US2007/147574A1 describes that a method for acquiring an image data set comprising energy integrating (EI) and energy discriminating (ED) data measurements is provided. The method comprises obtaining EI measurement data and ED measurement data during an acquisition cycle. The method then comprises combining the EI measurement data and the ED measurement data before, during or after reconstruction. Finally the method comprises performing reconstruction on the original or combined datasets to obtain one or more of an EI image and one or more ED component images.
WO2015/011587A1 describes that an imaging system includes a detector array that detects radiation traversing an examination region. The detector array includes at least a set of non-spectral detectors that detects a first sub-portion of the radiation traversing the examination region and generates first signals indicative thereof. The detector array further includes at least a set of spectral detectors that detects a second sub-portion of the radiation traversing the examination region and generates second signals indicative thereof. The imaging system further includes a reconstructor that processes the first and second signals, generating volumetric image data.
US2015/348258A1 describes that an apparatus is provided to reconstruct an image using combined third-generation energy-integrating computed tomography projection data and fourth-generation spectrally resolved computed tomography projection data. The apparatus includes processing circuitry configured to obtain first projection data representing projection data from an energy-integrating detector; obtain second projection data representing projection data from a photon-counting spectrally discriminating detector; and reconstruct a first combined-system basis image and a second combined-system basis image by solving a combined-system matrix equation using the first projection data and the second projection data.